The invention relates to an automatic gain control (AGC) device, and more particularly to an AGC device without being influenced by leakage current
An automatic gain control (AGC) device generates an output signal with desired amplitude by providing a properly stable gain control voltage for input signals with different amplitudes. Referring to FIG. 1, a conventional AGC device 10 includes a variable gain amplifier (VGA) 11, a top detector 12, a bottom detector 13, a subtractor 14, a target setting unit 15, a comparator 16, a charge pump 17, and a low pass filter (LPF) 18. The control device 10 receives an input signal Vin and outputs an output signal Vo from the VGA 11, whose gain, called the AGC gain, is controlled according to a gain control voltage Vg. The top detector 12 and the bottom detector 13 are used to detect a top voltage Vt and a bottom voltage Vb of the output signal Vo, respectively. The subtractor 14 calculates a voltage difference Vd between the top voltage Vt and the bottom voltage Vb, wherein the voltage difference Vd represents a peak-to-peak amplitude of the output signal Vo. Thereafter, the comparator 16 compares the voltage difference Vd with a target value Vs and then generates a comparison value. The comparison value is fed to the charge pump 17 for outputting a current signal, which is further fed to the LPF 18 for output the gain control voltage Vg to the VGA 11. The operation principle is further described in the following. When the comparison value is HIGH, it indicates that the voltage difference Vd is smaller than the target value Vs. At this time, the charge pump 17 charges the LPF 18 to increase the gain control voltage Vg such that the gain of the VGA 11 is increased, the dynamic range of the output signal Vo is increased, and the corresponding voltage difference Vd is also increased. Eventually, the voltage difference Vd will be equal to the target value Vs. On the contrary, if the voltage difference Vd is greater than the target value Vs, the charge pump 17 will discharge the LPF 18 to reduce the gain control voltage Vg. Therefore, the gain of the VGA 11 is dropped to decrease and the corresponding voltage difference Vd is also decreased. Eventually, the voltage difference Vd will be equal to the target value Vs.
The control device 10 utilizes a closed control loop to form an optimum gain control voltage for setting an optimum gain control. However, in some occasions (e.g., in a long-time seeking of an optical storage system), the amplitude of the input signal Vin to the VGA 11 is un-regular, then, it is desired that the operation of the control loop can be disabled temporally by trying holding the gain control voltage Vg for a period of time in order to avoid improper operations of the AGC device. Unfortunately, in practical, the gain control voltage Vg output from the LPF 18 can not be held due to the inherent leakage current of the LFP 18, which in turn still varies the AGC gain and may cause errors in system operations.